Abstract : Morphologically elastic fibers can be described as a fine fibrillar coating of a large amorphous core referred to as elastin. Elastin is an insoluble, highly cross linked and very hydrophobic protein with about 90% non polar amino acids and about 5% lysines. The insolubility of elastin is due to the presence of cross-links, primarily desmosine and isodesmosine, which are formed from four lysine residues, two each from two different peptide chains. The cross-linking sequences KAAAK and KAAK were observed to repeat at least six times in the soluble precursor protein, tropoelastin, which is comprised of 800-850 amino acids. Determination of the amino acid of porcine tropoelastin using tryptic peptides is 80% complete. A new mechanism of elasticity, a librational entropy mechanism, has been put forward to explain the elastic behavior of the polypentapeptide 2,3,4. The contrasts with the random chain network theory previously proposed for elastin.